It is well known that directionally, propeller efficiency improves as the propeller diameter increases and the speed (RPM) at which the propeller turns decreases. For this reason large cargo vessels, bulk carriers, tankers, etc., are commonly fitted with fuel efficient relatively large diameter slow-turning propellers, typically 20 to 30+ feet in diameter. However, the normal restraint on larger propeller diameter and therefore efficiency is that the tips of the propeller should not extend beyond or lower than the bottom keel of the vessel because of the risk of propeller damage upon inadvertent grounding of the ship in shallow waters. Furthermore, as the diameter of the ship propeller is increased, all the time maintaining the bottom tip path of the blades above the keel bottom, the upper tip path of the propeller will be higher and higher in the water approaching the light water line operating condition of the ship. Should the propeller tips come close to the waterline or extend above it, the propeller may lose more efficiency as a result of cavitation or ventilation than the efficiency gained by the increase in diameter.
Accordingly, in the design of ship propulsion systems, the selected propeller size and the chosen RPM for driving it at the ship's design speed represents a compromise or tradeoff between several variables, namely propeller efficiency in the loaded condition of a variable draft vessel such as a tanker vs. the propeller efficiency and its efficiency in the ballast or lightened draft operating condition.
The foregoing aspects of cargo vessel propulsion design are particularly important for cargo vessels which spend a high percentage of their operating time in lightly loaded condition, or in ballast which usually means empty of cargo returning to a loading port for the acceptance of another cargo. Such return ballast voyages can be accomplished most efficiently (requiring minimum propulsion power) if the vessel is at its minimum draft. However, as previously related, a constraint on the minimum draft will always be the reduced efficiency of the propulsion system, as the ship becomes higher and higher in the water, due to the fact that a portion of the propeller arc (the propeller tips) will break the water surface. Accordingly, it has been the practice to keep the ship sufficiently down with ballast water during such return voyage to keep the propeller fully submerged, or alternatively operate the ship with greater trim by the stern (greater immersion of the stern and less of the bow), or alternatively to lower the diameter of the chosen propeller for the ship design so that it may operate in lighter ballasts without breaking the water surface. The first and second of the foregoing alternates restrict the potential for operating with lighter ballast or minimum trim which inherently are more fuel efficient, requiring less horsepower to achieve a given ship speed. The third alternate of lower propeller diameter as explained earlier lowers the propulsion efficiently resulting again in higher fuel consumption.